1. Field of the Invention
This invention relates generally to the lamina-by-lamina formation of three-dimensional objects through application of the principles of stereolithography, and more specifically, to the automatic detection of surface features of selected laminae of the three-dimensional objects.
2. Description of Related Art
Several building techniques exist for building three-dimensional objects in layers (e.g. laminae). One such technique is stereolithography, which is described in U.S. Pat. No. 4,575,330 (hereinafter referred to as the '330 patent). According to the principles of stereolithography, a three-dimensional object is formed layer-by-layer in a stepwise fashion out of a material capable of physical transformation upon exposure to synergistic stimulation (e.g., fluid or fluid-like material such as a photopolymer, sinterable powder, or bindable powder). In one embodiment of stereolithography, as shown in FIG. 1, layers of liquid photopolymer are successively formed and selectively transformed into solidified laminae which are adhered to adjacent previously formed laminae at the working surface of a volume of the liquid photopolymer.
In building a three-dimensional object and more particularly in the step of forming layers of flowable material (e.g. liquid photopolymer material) adjacent to any previously formed lamina, the goal is to form coatings of desired thickness (e.g. uniform thickness of 2-10 mils) such that the unsolidified material has a desired surface shape (e.g. planar surface) in preparation for forming a next lamina (i.e. layer of the object) from data which is descriptive of the three-dimensional object and which is correlated to the desired thickness and surface shape (e.g. planar data representing a particular cross-section of the object having a desired thickness). Since it is typical to work with planar layers, it is important that the surface level of building material in the working area (i.e. the area that can or will be exposed to the prescribed stimulation) be at the same level. If during formation, the configuration or geometry of the desired object results in regions of unsolidified material that are isolated from or poorly connected to other regions of unsolidified material, there may be a difficulty in causing those effectively isolated regions to attain the same surface level in a time period thought to be reasonable or optimal for the recoating process. This has been found to be particularly problematic when an isolated region results from wall-like structures which have upper surfaces close to the desired surface level of the unsolidified material (e.g. 1 layer thickness--2 to 10 mils) and a floor-like structure which is further under the desired surface level. These particularly problematic regions have come to be known as trapped volumes. It is further known that trapped volumes can effectively exist even though complete isolation does not exist. For example effective trapped volumes can exist even though gaps, holes or breaches may exist in the side walls or floor of the solidified material which bounds the region of unsolidified material. In other words, an effective trapped volume can exist whenever two bodies (i.e. regions) of unsolidified material are separated by a restricted flow path that limits sufficient flow from one region to the other such that a uniform surface level between the two regions is not naturally reached within a desired amount of time.
Problems associated with trapped volume regions are discussed in U.S. Pat. No. 5,258,146. Trapped volumes can lead to difficulties in using a doctor blade and other recoating devices. Typically, in the process of forming a coating over a just formed lamina, it is desirable to form a coating of excess thickness over the lamina which can be swept down to the right thickness by a smoothing device (e.g. a flexible or rigid doctor blade, a flexible or rigid rake, a brush, etc.). In fact the height of the excess material in front of a doctor blade typically grows as the blade is swept across the last solidified layer as more and more material is removed from the region just swept by the blade. The existence of a large bulge of material in front of the blade can be problematic when sweeping over a trapped volume. When a large bulge of material is in front of a smoothing device and a trapped volume is encountered, material in the bulge may flow backwards underneath the blade and thereby disrupt the desired layer thickness of the smooth layer which was hoped to be formed in the wake of the blade. A height differential in front of and behind the blade creates a pressure difference between the front and the back of the blade. This situation can cause a driving force which can cause the resin to flow dependent on the viscosity of the resin and the size of the flow paths available to the resin. In a trapped volume situation, the most open flow path may be for the resin to flow under and behind the doctor blade as opposed to flowing ahead of the blade. As a result, a substantial bulge may remain within the trapped volume region as opposed to being swept ahead of the blade and out of the trapped volume region. Therefore, when the doctor blade finishes the sweeping over the trapped volume region, the amount of resin removed from the trapped volume may be far less than desired.
This problem is illustrated with FIG. 2a which shows recoating device (e.g. blade) 20 in the middle of a sweep from left to right as indicated by arrow 26. Prior to beginning the sweep from left to right, part (i.e. object) 24 has been over coated with unsolidified material having an excess thickness. During the sweeping step, the separation of the blade and the desired working surface is known as the blade gap, while the separation of the blade from the upper surface of the last solidified layer is known as the blade clearance. For simplicity in this example, the blade gap is assumed to be zero and the blade clearance is assumed to be one layer thickness. In FIG. 2a the blade clearance is indicated to be a distance 23. As indicated by reference numeral 21, the resin surface in the wake of the blade is shown as being smoothed and trimmed to the desired level, while a bulge of excess resin, identified by reference numeral 22, is formed in front of the blade. In fact as the blade sweeps, the bulge in front of the blade gets larger since the material removed from the regions that are now behind the blade is added to those regions remaining in front of the blade. To attain its goal, blade 20 must sweep the excess material away from the object; however, this goal may be thwarted when the blade encounters a trapped volume of resin 25. Although the pressure differential between the material above the desired surface level (i.e. the material in bulge 22) and remaining material, will cause material in the bulge to flow to other regions, the rate of material transfer depends on a number of factors including the size of the flow paths available. As can be seen, flow path 27, within trapped volume 25, is much larger than flow path 28 over the right-hand wall 27. As such, material in bulge 22 is much more likely to flow to the region behind the blade than to flow through path 28. This disrupts the formation of smooth surface 21. In other words, this back flow of material results in insufficient material being removed from the trapped volume region, thereby causing the actual surface level to be too high within the trapped volume and too low outside the trapped volume. This effect is less pronounced where the blade is traveling over a flat, horizontal surface of a part, since the flow path in front of the blade is larger than the flow path underneath the blade 20.
Automatic vent or drain generation, as discussed in International Publication No. WO 95/29053, helps eliminate trapped volume problems, but does so by inserting holes into what should have been solid regions of the object. After object formation these holes must be filled to achieve object integrity. A solution which does not exchange one problem (trapped volume recoating issues) for another problem (unwanted holes in the object) is desired.
In a stereolithography apparatus ("SLA") using an applicator blade, a different type of problem exists. The applicator blade may use an opposite approach to recoat than that of the doctor blade. In a stereolithography apparatus using this technique and shown in FIG. 2b, after the last formed object cross section 30 has been formed by selectively exposing the building material to synergistic stimulation, the object 31 is dipped into the building material to a depth of approximately one layer thickness, or other desired thickness, below the desired working surface 32 of the building material 33. During the exposure process, applicator 34 is at least partially filled with material 33 and after the exposure process, applicator 34 is swept at or slightly above the desired working surface 32 while dispensing material from opening 36 to form an adjacent building material layer 37.
Because there is typically no process to form an initial coating of excess thickness (e.g. deep dipping where the object is dipped into the building material by more than one layer thickness) in a typical layer formation process using the applicator blade (i.e. a material dispensing device), not only must the applicator supply resin over the regions solidified in association with the formation of the last lamina but it must also add resin into the trapped volume as it moves across that region. If the applicator blade fails to provide exactly the right amount of resin into the trapped volume region, any excess liquid may not flow from the trapped volume region quickly enough or any shortage of resin might not be filled in quickly enough due to the flow restrictions inherent therein. Though, these problems may be minor when considering the formation of a single layer, they can be problematic, if not catastrophic, to part building when a plurality of adjacent layers continue the formation of a trapped volume, whereby these errors can be accumulated. Consequently, trapped volume regions are problematic to defining generalized optimized recoating styles when using either a doctor blade or an applicator blade.
Another parameter which can be varied to provide a uniform coating over a lamina is the speed of the blade. The speed may be varied between multiple sweeps over the same lamina or it may be varied between recoating processes over different laminae. It is even possible to vary the speed when recoating over different portions of a single lamina. When the blade is passing over a large, horizontal flat area of a part, if the blade is traveling too fast, too much resin may be swept away because of drag. In some instances drag may be so severe over flat areas, that substantially all liquid resin is swept away by the blade. Therefore, over large flat areas, it may be desirable to slow the blade speed so that the above problem does not occur.
On the other hand, over trapped volume regions, if the blade moves too slowly, this may give the resin in the bulge more time to flow underneath the blade. Therefore, over trapped volumes, it may be desirable to increase the speed of the blade so that resin does not have time to flow underneath.
In addition, object cross-sections which have narrow widths of solidified material can be efficiently coated over with different recoating styles (e.g. recoating techniques and/or recoating parameters) than are necessary with cross-sections having large solidified areas. Furthermore, not only is the structural configuration of the last solidified lamina critical to recoating, the structural configuration of laminae located within 0.5 to 2 mm or more below the target surface may also play a critical role in determining how to form coatings in optimal time, with optimal uniformity, and/or with desired thickness.
It has been found that a single set of recoating styles (i.e. recoating techniques and/or recoating parameters) cannot generally be optimally used to handle both trapped-volume regions and non-trapped-volume regions (such large solidified flat surface features). In fact, if a given layer contains both a trapped-volume region and a non-trapped-volume region (e.g. a large flat region), a different recoating style may be more optimal than that selected to optimize the process when either of these features exist alone. In existing stereolithography systems the selection and application of recoating styles can be done in two ways: 1) a single set of recoating parameters may be selected to form an object wherein the recoating parameters are not optimized for the most accurate and/or fastest formation of each layer thereby causing a reduction in through put of the system or loss of accuracy in the final object formed, or 2) recoating parameters can be manually selected on a range-by-range basis (i.e. a vertical level by vertical level basis) by the system operator based on his/her experience and understanding of the object to be formed, thereby resulting in more or less optimization in throughput and accuracy. The need for a more fully automated system for selecting recoating styles has been long felt in the art.
Due to the above phenomena, there is a long existing need in the stereolithography art for a more reliable, optimized, automated method and apparatus for forming layers of flowable building material in preparation for forming subsequent layers of an object being built. As a first step in producing such reliable, optimized, automated methods and apparatus it is important to accurately determine the presence of surface features of one or more previously formed object layers (e.g. large flat solidified regions and trapped volumes for each lamina.
All the U.S. patents and U.S. applications referred to herein are hereby incorporated by reference as if set forth in full herein.